1. Field of the Invention
The present invention relates to a voltage supply circuit supplying a bit-line precharging voltage in a semiconductor memory, such as a DRAM (Dynamic Random Access Memory) or the like.
2. Description of the Related Art
In general, a semiconductor memory, such as the DRAM or the like, has an internal power supply circuit that uses an external power supply voltage supplied through an external terminal to generate a plurality of internal power supply voltages. For example, the internal power supply circuit includes a plurality of voltage supply circuits, which supply a bit-line precharging voltage, a memory-cell plate voltage, a word-line activating voltage, a bit-line restoring voltage, and the like, respectively.
Here, the operation of a semiconductor memory, such as the DRAM or the like, will be briefly described. When a semiconductor memory changes from a standby state to an active state, a precharging control signal to a precharging circuit (a circuit that connects pair of bit lines to a precharging voltage line) and an equalizing control signal to an equalizing circuit (a circuit that connects the pair of bit lines with each other) are inactivated, and then a word line is activated. Accordingly, a precharging operation and an equalizing operation for the pair of bit lines stop, and a potential difference occurs between the pair of bit lines due to an electric charge accumulated in a memory cell. By potential difference being amplified by a sense amplifier, the voltage of one of the pair of bit lines and the voltage of the other bit line are set to a restoring voltage and a ground voltage, respectively.
Subsequently, when the semiconductor memory changes from the active state to the standby state, the word line is inactivated, and then the precharging control signal and the equalizing control signal are activated. Accordingly, the precharging operation and the equalizing operation for the pair of bit lines restart. Since one of the pair of bit lines and the other bit line have substantially the same electric load capacitance, the voltage of one of the pair of bit lines and the voltage of the other bit line are set to approximately half of the restoring voltage by the equalizing operation.
When the precharging voltage is set to a half of the restoring voltage, in the precharging operation after the restoring operation as described above, a current which has to be supplied by a voltage supply circuit for a precharging voltage barely exists. Further, when the precharging voltage is set to a half of the restoring voltage, in the precharging operation after a read operating or a write operation, the current which has to be supplied by the voltage supply circuit for a precharging voltage barely exists. As such, when the current which has to be supplied by the voltage supply circuit for a precharging voltage is constantly small regardless of the operation state of the semiconductor memory, drivability (current supply capacity) of the voltage supply circuit for a precharging voltage could be small.
In general, a push-pull-type voltage supply circuit is used as the voltage supply circuit for a precharging voltage. In the push-pull-type voltage supply circuit the output voltage is set substantially constant by an output node being connected to a restoring voltage line or a ground line through output transistors when an output voltage is deviated from a predetermined voltage range (dead zone). In such a push-pull-type voltage supply circuit, the output voltage is not influenced by threshold voltages of the output transistors, like a source-follower-type voltage supply circuit (for example, see Japanese Unexamined Patent Application Publication No. 2001-325792). For this reason, the push-pull-type voltage supply circuit can set the output voltage with higher precision, as compared to the source-follower-type voltage supply circuit. Further, the change in an output current in a voltage region neighboring the dead zone is precipitous in the push-pull-type voltage supply circuit. For this reason, the change in the output voltage depending on the output current is smaller in the push-pull-type voltage supply circuit, as compared to the source-follower-type voltage supply circuit.
On the other hand, there is a case in which the precharging voltage VPR is set lower than a half of the restoring voltage VBLH in order to improve a margin for reading out data of the sense amplifier. In this kind of case, the current which has to be supplied by the voltage supply circuit for a precharging voltage temporarily increases in the precharging operation after the restoring operation. The current IVPR in this case is expressed using load capacitance of one of the pair of bit lines CBL, the number of sense amplifiers to be activated NSA, and the activation period of each of the sense amplifiers TCYC, as shown in the following equation (1).IVPR={(VBLH/2−VPR)·2·CBL·NSA}/TCYC  (1)
Further, when a voltage supply circuit for a restoring voltage is constituted using a source-follower circuit of nMOS transistors, the voltage of one of the pair of bit lines at the time of the end of the restoring operation increases as the restoring operation period extends. Even in this kind of case, the current which has to be supplied by the voltage supply circuit for a precharging voltage temporarily increases in the precharging operation after the restoring operation. The current IVPR in this case is expressed using the voltage of one of the pair of bit lines at the time of the end of the restoring operation VBLX, as shown in the following equation (2).IVPR={(VBLX/2−VPR)·2·CBL·NSA}/TCYC  (2)
As such, the voltage supply circuit for a precharging voltage is required to have large drivability when the current, which has to be supplied by the voltage supply circuit for a precharging voltage, temporarily increases according to the operation state of the semiconductor memory. In order to increase drivability of the push-pull-type voltage supply circuit, it is preferable to increase the cannel widths of the output transistors. However, when the channel widths of the output transistors increase, the load capacitance of the output terminal of each of differential amplifiers, which outputs control signals to the output transistors, increases. For this reason, the response speed against the change of the output voltage is lowered.
Further, in the push-pull-type voltage supply circuit, a feedback loop is formed by a two-stage amplifying circuit. Accordingly, when the channel widths of the output transistors increase, the stability against oscillation lowers, and thus causes oscillation to easily occur. The lowering of the response speed against the change of the output voltage and the lowering of stability against oscillation can be avoided by increasing bias currents of the differential amplifiers. However, when the bias currents of the differential amplifiers are constantly increased regardless of the operation state that the semiconductor memory is in, power consumption in the standby state of the semiconductor memory increases.